System and method for coherent detection with digital signal procession

ABSTRACT

Methods and apparatus to realize high spectral efficiency in optical signals transmitted over long distances.

FIELD OF THE INVENTION

The present invention relates to communication systems, andparticularly, coherent detection with digital signal processing.

BACKGROUND

Increasing bandwidth demand has been driving communication systems tohigher capacities. Therefore, there is a strong motivation to enhancespectral efficiency to increase the total capacity. Employing opticalorthogonal frequency division multiplexing (O-OFDM) modulation totransmit signals can realize high-spectral efficiency and long distancetransmission. To achieve high receiver sensitivity with coherentdetection based on digital signal processing, the bandwidth of theanalog to digital converter (ADC) and the sample rate may be high.Usually, the ADC bandwidth may have two times of the bit rate of thesignal, and the sampling rate may be four times of the bit rate. Forexample, if each subcarrier of the OFDM signal is 25 Gbaud QuadraturePhase Shift Keyed (QPSK), the ADC bandwidth should be 50 GHz and thesample rate should be 100 GSa/s to obtain optimum results. However, anADC with these specifications may not be commercially available.Therefore it would be advantageous to reduce the ADC bandwidth andsample rate while maintaining the same performance.

SUMMARY OF THE INVENTION

Aspects of the present invention employ optical orthogonal frequencydivision multiplexing (O-OFDM) to transmit signals realizing highspectral efficiency over long distances.

Aspects of the present invention include apparatus and methods fortransmitting and receiving signals in a communication system. Amulticarrier generator generates a multicarrier signal. An opticaldemultiplexer separates the multicarrier signal into separate subcarriersignals. Phase and QPSK modulators modulate signals from the separatesubcarrier signals. An optical multiplexer combines the modulatedsubcarrier signals into a multiplexed signal. The multiplexed signal isthen transmitted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of a transmitter and receiveraccording to aspects of the present invention.

FIG. 2 illustrates a schematic diagram of digital signal processing fora coherent receiver according to aspects of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Aspects of the present invention employ optical orthogonal frequencydivision multiplexing (O-OFDM) to transmit signals realizinghigh-spectral efficiency over long distances.

FIG. 1 illustrates a schematic diagram of a transmitter and receiveraccording to aspects of the present invention. A laser 101 generates acontinuous lightwave. The laser 101 may be a distributed feedback typelaser diode (DFB-LD), which may have a wide line width. For a 100 Gbit/sQPSK for example, a line width smaller than 2 MHz may be used, althoughin embodiments line widths greater than 2 MHz may also be sufficient.Alternatively, the laser source 101 may be a tunable external laser witha narrow line width and low phase noise which may be preferred for highlevel modulation format signals. A multicarrier generator 102 receivesthe lightwave and generates a multicarrier signal. This multicarriersignal may be generated by a few different schemes, such as cascadedphase and intensity modulators driven by a sinusoidal wave source, orcascaded phase modulators. In an embodiment, ten subcarriers with fixedfrequency spacing off may be generated, although another number ofsubcarriers may be generated.

To separate the optical subcarriers for routing to respective ports, anoptical demultiplexer may be employed 103. This optical demultiplexer103 may be an array waveguide grating, an optical fiber Bragg grating,or other optical demultiplexer as known in the art. Each subcarrier fromthe respective output ports of the optical demultiplexer 103 may bemodulated by using an optical I/Q modulator 104. In particular, theoptical I/Q modulator 104 generates QPSK signals. These QPSK signals mayhave a non-return-to-zero or return-to-zero pulse shape. This signal maybe a polarization multiplexed signal. The optical I/Q modulator 104 maybe driven by four individual data signals, that is, In Phase (I) andQuadrature Phase (Q) for X polarization, and I and Q for Y polarization.The baud rate of the I or Q signals may preferably be f Gbaud/s.

An optical multiplexer 105 with a 3 dB bandwidth off GHz may be used tocombine the modulated signals from the optical I/Q modulator 104 assubchannels. The optical multiplexer 105 may be a regular WDM filter, aWDM coupler, an array waveguide grating (AWG), or other optical filterto combine all of the subchannels. An optical amplifier 106 may be usedto compensate for subsequent transmission fiber loss. This opticalamplifier 106 may be an Erbium doped fiber amplifier, Raman amplifier,or other amplifier to provide gain as is known in the art. Themultiplexed signal may then be transmitted over a fiber 107. The fiber107 may be any transmission fiber. In embodiments, optical amplifier 106may alternatively or additionally be placed at the receiving side oftransmission fiber 107.

The transmitter disclosed in the foregoing is different fromconventional optical OFDM signal generation at least in that, incontrast to the prior art, there is no time synchronization between thetransmitter and the receiver. Moreover, optical couplers are not used tocombine the subchannels as in the prior art. Instead, the disclosedtransmitter uses an optical multiplexer such as arrayed waveguidegrating to combine subchannels.

On the receiver side, coherent detection based on digital signalprocessing is used. The coherent detection technique employs the use ofan optical local oscillator 108, a 90 degree hybrid 109, four balancedreceivers, ADC chips and ASIC chips for digital signal processing. Thefrequency of the optical local oscillator 108 is preferably the same asthe frequency of one of the subcarriers. The local oscillator 108 may bea distributed feedback laser (DFB) or an external cavity laser with aline width preferably smaller than a few MHz. The 90 degree hybrid 109may be a regular optical 90 degree hybrid to demultiplex the I and Qsignal. Digital coherent detection block 110 includes balanced orunbalanced photodiodes, high speed ADC and other electrical componentssuch as ASIC, FEC, and the like.

The receiver is different from prior art arrangements at least in thatit does not require wideband ADC chips with high sampling rate to detectthe received signal. Instead, commonly available ADC chips with lowbandwidth may be used. In an exemplary embodiment, for subchannelspacing f GHz, an ADC bandwidth of about 0.5 f GHz is sufficient, and asampling rate of about 1.5 f GSa/s or more is sufficient. Moreover, anadditional DSP with one post filter and MLSE are employed for datadetection.

FIG. 2 illustrates a schematic of digital signal processing (DSP) for acoherent receiver with post filter and maximum likelihood sequenceestimation (MLSE). A compensation module 200 may correct an I/Qimbalance of the received signal. A dispersion compensating unit 201 maycompensate for chromatic dispersion. Sampling unit 202 samples andresamples the signal, and each bit is sampled twice. Through the use ofadaptive equalizers 203, a polarization demultiplexer is realized thatgenerates polarization demultiplexed signals. An offset module 204compensates for a frequency offset of the demultiplexed signals in orderto improve the quality of communication. Phase module 205 phasecompensates the demultiplexed signal. A filter 206 post filters thephase compensated signal. The filter 206 may be a 2 tap filter. MLSE,which may be two state, is applied to the filtered signals to recoverdata in the signals in data estimator 207. A bit error rate may becalculated in BER calculator 208.

The foregoing discloses and describes novel methods and systems forcoherent detection with digital signal processing. In the transmitter,there are several subchannels. Each subchannel has a channel spacing offGHz; each subchannel carries f Gbaud QPSK signal. After opticalmultiplexing and signal transmission in optical fiber, coherentdetection is employed in the receiver, with DSP to detect the signal.This DSP includes commonly available DSP hardware, with additional postfilter and MLSE processing.

It should be understood that the methods and devices of the presentinvention may be executed employing machines and apparatus includingsimple and complex computers. Moreover, the architecture and methodsdescribed above can be stored, in part or in full, on tangible machinereadable media. For example, the operations of the present inventioncould be stored on media such as magnetic disks or optical disks, whichare accessible via a disk drive (or computer-readable medium drive).Alternatively, the logic to perform the operations as discussed above,could be implemented in additional computer and/or machine readablemedia, such as discrete hardware components as large-scale integratedcircuits (LSI's), application-specific integrated circuits (ASICs),firmware such as electrically erasable programmable read-only onlymemory (EEPROMs); and the like. Implementations of certain embodimentsmay further take the form of machine-implemented, includingweb-implemented, computer software.

While aspects of this invention have been shown and described, it willbe apparent to those skilled in the art that many more modifications arepossible without departing from the inventive concepts herein. Theinvention, therefore, is not to be restricted except in the spirit ofthe following claims.

What is claimed is:
 1. A method of generating a wide bandwidthmultiplexed optical signal and transmitting it over an optical fiber,the method comprising: generating, by a laser, a continuous lightwave;generating, from the continuous lightwave, a multicarrier signal havinga fixed channel spacing off GHz; separating the multicarrier signal intoa plurality of optical subcarriers; routing each of the opticalsubcarriers to a respective I/Q modulator and modulating each of thesubcarriers to carry f Gbaud of data as a QPSK signal by thecorresponding I/Q modulator; combining, in an optical multiplexer, themodulated optical subcarriers into a multiplexed optical signal; andtransmitting the multiplexed optical signal with no synchronizationinformation over an optical fiber, so that blind data detection isrequired at the receiving end of the communication.
 2. The method ofclaim 1, wherein the continuous lightwave has a line width of about 2MHz.
 3. The method of claim 1, wherein the laser is a tunable externallaser with a line width narrower than 2 MHz and low phase noise.
 4. Themethod of claim 1 wherein the multicarrier generator comprises cascadedphase and intensity modulators driven by a sinusoidal wave source. 5.The method of claim 1, wherein at least ten subcarriers are generated.6. The method of claim 1, wherein the QPSK signals are polarizationmultiplexed together.
 7. The method of claim 1, wherein the optical I/Qmodulator is driven by four data signals, including in phase (I) andquadrature phase (Q) for X polarization and I and Q for Y polarization.8. The method of claim 1, wherein the I signals and Q signals are eachmodulated to carry f Gbaud/s.
 9. The method of claim 1, wherein themultiplexer is one of a regular WDM filter, a WDM coupler, an arraywaveguide grating, and an optical fiber Bragg grating.
 10. The method ofclaim 1, wherein the optical multiplexer has a 3 dB bandwidth of −f GHz.11. The method of claim 1, wherein the modulated signals arepolarization multiplexed.
 12. The method of claim 1, further comprisingamplifying the multiplexed signal before it is transmitted to compensatefor transmission loss in the optical fiber.
 13. A method of blinddetecting of data contained in a wide bandwidth multiplexed opticalsignal transmitted over optical fiber, the method comprising: receiving,by a coherent optical receiver, a wide bandwidth multiplexed opticalsignal from an optical fiber; applying a local oscillator having afrequency substantially equal to a subchannel spacing f of the receivedmultiplexed optical signal to the received multiplexed optical signal toobtain a plurality of subcarriers; polarization demultiplexing an Xsignal and a Y signal from each of the subcarriers; demultiplexing an inphase (I) signal and a quadrature phase (Q) signal from each of the Xand Y signals; and coherently detecting data carried by each of the Iand Q signals using: an ADC with a bandwidth of about 0.5 f; a signalsampler with a sampling frequency of about 1.5 f, and a digital signalprocessor DSP configured to: condition the sampled signals; and applymaximum likelihood sequence estimation (MLSE) to the conditioned signalsto estimate data carried on each of the I and Q signals.
 14. The methodof claim 13, further comprising correcting an I/Q imbalance of thereceived signal.
 15. The method of claim 13, further comprisingcompensating for chromatic dispersion in the received signal.
 16. Themethod of claim 13, further comprising compensating for a frequencyoffset of the demultiplexed signals.
 17. The method of claim 13, furthercomprising phase compensating the demultiplexed signal.
 18. The methodof claim 17, further comprising post filtering the phase compensatedsignal.
 19. The method of claim 13, further comprising calculating a biterror rate of the estimated data.
 20. An apparatus for detecting datacontained in an optical signal received from an optical transmissionfiber, comprising: a local oscillator having a frequency substantiallyequal to a subchannel spacing f separating a plurality of subchannelscontaining in the received optical signal, to obtain a plurality ofsubcarrier signals; a polarization demultiplexer to obtain an X signaland a Y signal from each of the subcarrier signals; an OFDMdemultiplexer to obtain an in phase (I) signal and a quadrature phase(Q) signal from each of the X and Y signals; an ADC with a bandwidth ofabout 0.5 f; a signal sampler with a sampling frequency of about 1.5 f;and a digital signal processor DSP configured to condition the sampledsignals and apply maximum likelihood sequence estimation (MLSE) to theconditioned signals to estimate data carried on each of the I and Qsignals.